Problem: Solve for $z$ : $9 = z + 27$
Answer: Subtract $27$ from both sides: $ \begin{eqnarray} \\ \hphantom{9 {- 27}} &\hphantom{=}& \hphantom{z} \\ 9 &=& z + 27 \\ \\ {-27} && {-27} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 9 {- 27} &=& z \\ \end{eqnarray} $ Simplify. $ z = -18$